Semiconductor laser diodes are generally known and disclosed, for example, in Chapter 12 of Sze, Physics of Semiconductor Devices, 2nd ed. pp. 681-742 (1981). Specifically, a II-VI semiconductor laser diode is disclosed in U.S. Pat. No. 5,513,199 issued to Haase et al.
Typically, semiconductor laser diodes include parallel facets that are formed when a plurality of semiconductor laser diodes are formed by cleaving a semiconductor crystal along the crystal's natural cleavage planes. The facets help confine light that is emitted by a pn junction located within the semiconductor laser diode. The facets help confine the emitted light by reflecting back into the semiconductor body a fraction of the light that otherwise would exit the semiconductor body. This reflection of the emitted light promotes a condition where the reflected light oscillates within the semiconductor body. This oscillation condition is necessary for the semiconductor to operate as a laser.
Facet degradation processes are well known in III-V semiconductor laser diodes and are associated with oxidation of the facet. Oxidation is a condition where an oxide film grows on the facet when the facet is exposed to the ambient environment. It is believed that oxidation of the facet is enhanced by laser light that is reflected by the facet. When the facet oxidizes, elements of the semiconductor (e.g., Ga or As) are nonuniformly removed from the semiconductor body to form the oxide film. This leaves behind defects in the semiconductor body which cause a portion of the emitted light to be absorbed in a process referred to as nonradiative recombination. In nonradiative recombination, the energy from the absorbed light is dissipated in part in the form of heat. When the laser diode is operated at high output powers, the temperature increase at the facet due to nonradiative recombination can be large. If the temperature at the facet exceeds the melting point of the semiconductor material used to form the laser, rapid destruction of the facet occurs, which prevents the laser diode from operating. This destruction of the facet is referred to as catastrophic optical damage (COD). When COD occurs, fractures appear on a portion of the facet and a molten region penetrates into the semiconductor laser from the facet. The output power at which COD occurs tends to decrease as the laser ages due to increased oxidation of the facet.
In III-V semiconductor laser diodes, facet degradation is typically minimized by coating the facet with a dielectric material such as Al.sub.2 O.sub.3 which effectively suppresses oxidation of the facet. This approach is described generally in Fukuda, Reliability and Degradation of Semiconductor Lasers and LEDs, pp. 134-136 (1991).
Conventional II-VI semiconductor laser diodes also exhibit a type of facet degradation. A gradual darkening of the semiconductor laser near the facet has been observed. After many minutes to an hour of operating a II-VI semiconductor at high output powers, it has been observed that this darkened region can extend up to 10 .mu.ms from the facet. Eventually dark line defects usually grow out of the darkened region and result in functional destruction of the laser.
For II-VI semiconductor laser diodes, however, coating the facet with a dielectric, as is done with III-V semiconductor devices, is not sufficiently effective to reduce facet degradation to acceptable levels. Therefore, a new approach is needed to reduce facet degradation in II-VI semiconductor laser diodes.